The present invention relates to disc drive spindle motors and, more particularly, to a spindle motor having a tuned stator resonant frequency.
Disc drive data storage devices, known as "Winchester" type disc drives, are well known in the industry. In a Winchester disc drive, digital data is written to and read from a thin layer magnetizable material on the surface of a rotating disc. Write and read operations are performed through a transducer which is carried on a slider body. The slider and the transducer are sometimes collectively referred to as a head, and typically a single head is associated with each disc surface. The heads are selectively moved under control of electronic circuitry to any one of a plurality of circular, concentric data tracks on the disc surface by an actuator device. In the current generation of disc drive products, the most commonly used type of actuator is a rotary moving coil actuator.
The discs are typically mounted in a "stack" on the hub structure of a brushless DC spindle motor. The rotational speed of the spindle motor is precisely controlled by motor drive circuitry which controls both the timing and the power of commutation pulses directed to the stator windings of the motor. Traditional spindle motor speeds have been in the range of 3,600 RPM. Current technology has increased spindle motor speeds to 10,000 RPM and above.
Analysis of the various types of disc drives has brought to light several different modes of acoustic noise generation which are attributable to the spindle motor and its control logic. One mode of noise generation is sympathetic vibration of the disc drive housing in response to the rotating mass of the spindle motor. Another mode of acoustic noise generation is electromagnetic disturbances caused by the excitation of the stator mass by the application and removal of the commutation pulses that are used to drive the motor and control its speed. The commutation pulses are timed, polarization-selected DC current pulses which are directed to sequentially selected stator windings. The rapid rise and fall times of these pulses act as a striking force and set up sympathetic vibrations in the stator structure. Interaction of resonant vibrational frequencies of the stator and its support structure with the fundamental forcing frequencies of the commutation pulses and their harmonics is a well known contributor to disc drive acoustic noise, and especially pure tone vibrations.
Several attempts have been made to tune the stator resonant frequency away from the fundamental forcing frequencies and their harmonics. For example, features have been machined into the spindle shaft of an in-hub spindle motor or into the stator mounting boss of an under-the-hub spindle motor. These "designed-in" machined features are effective at tuning the stator torsional resonant frequency, but cannot easily account for natural part variations associated with high volume manufacturing of spindle motor components such as shafts, stators and motor bases.
Improved methods of tuning the stator resonant vibrational frequency in high volume manufacturing processes are desired.